1. Technical Field
This disclosure is generally related to flexible antennas having materials that leak radio frequency (RF) energy and, in particular, to an antenna having an impedance matching section.
2. Description of Related Art
A typical antenna suitable for attaching to clothing 100 is a patch or dipole-based antenna or similar as shown in FIG. 1. Radiating coaxial antennas having helical outer conductors are typically used for communication in a highway tunnel. Radiating coaxial cables have been used within tunnels and buildings where electromagnetic propagation is difficult and where wireless communication is desired over a wide area as disclosed by Suzuki, T.; Hanazawa, T.; Kozono, S., “Design of a tunnel relay system with a leaky coaxial cable in an 800 MHz band land mobile telephone system,” Vehicular Technology, IEEE Transactions on, Volume 29, Issue 3, August 1980, Page(s): 305-316. Typically, the radiation will leak away from the coaxial cable, from openings in the outer conductor of the cable as disclosed by Thomas, R. W.; Brown, J. S, “Radiax™, a new radiating coaxial cable,” Vehicular Technology Conference, 1971. 22nd IEEE Volume 22, 7-8 Dec. 1971 Page(s): 430-439. Such antennas are sometimes termed as “leaky” antennas. The leaky coaxial cable is inherently a broadband traveling wave antenna, since the dominant feed mode is transverse electromagnetic mode (TEM).
Such radiating coaxial antennas tend to be long antennas that leak electromagnetic energy from a coaxial cable at a slow rate, owing to a poor radiating efficiency. An existing antenna has a leakage rate of 3 dB per 100 feet. In particular, patch and dipole-based antennas tend to be narrowband and have a limited field of view (FOV), typically a maximum of 100° as described in “Broadband Microstrip Antennas,” G. Kumar, K. P. Ray, Artech House, 2003, in Table 2 on page 43. Further, because of dimensions and/or layout, such antennas are ill-suited for attaching to clothing.
Another leaky coax is disclosed by Henry Ryman in “Radiating Coaxial Cable with Outer Conductor Formed by Multiple Conducting Strips,” U.S. Pat. No. 5,936,203, Aug. 10, 1999. The '203 patent teaches a use of a single or a double wound helical strip to form a leaky wave outer conductor shield for a radiating coaxial cable. Examples of openings permitting leakage of radio frequency energy from the cable are shown in FIGS. 2 and 3 discussed below.
FIGS. 2a and 2b show a segment 200 of Ryman's single helix leaky cable. A conductor 205, having a length L, is enclosed in a dielectric 215. A conducting film or strip 225 is wound in a single helix on the dielectric 215. The conducting film or strip 225 has a width W and a pitch P. An impedance of the existing antenna is controllable by a variation of the width W of the conducting film or strip 225.
FIGS. 3a and 3b show a segment 300 of Ryman's double helix leaky cable. A conductor 305, having a length “L,” is enclosed in a dielectric 315. A conducting film or strip 325 is wound in a first helix. An additional conducting film 335 is counterwound in a second helix on the dielectric 315. The conducting film or strip 325 has a width W1 and a pitch P1. The additional conducting film 335 has a width W2 and a pitch P2. It should be noted here that W1 and W2 may be the same or different depending on a given application. Similarly, P1 and P2 may be the same or different depending on a given application. Further, P1 may be different from W1 and P2 may be different from W2. Any of W1, W2, P1, and P2 may be varied to arrive at a desired impedance or radiation leakage rate. It may be pertinent to note that the leakage radiation occurs because of a discontinuity.
The use of multiple quarter-wave transmission line sections of prescribed impedances is taught by Cohn, S. B., “Optimum Design of Stepped Transmission Line Transformers,” IRE Trans. Microwave Theory Tech., Vol. MTT-3, pp. 16-21, April, 1955.
The related application identified above teaches a radiating coaxial cable transmission line that may be used as an antenna and incorporated into a garment. Mechanisms are incorporated into the antenna for boosting the rate of conversion of bifilar mode to monofilar mode.